In conventional air cleaners such as used on the internal combustion engines in passenger cars and trucks, it is common practice to provide one or more drain holes in the air cleaner casing between the air inlet and the upstream side of the air filter. The purpose of these holes is to drain any liquid collected on the bottom of the cleaner casing from the air passing through the filter and onto the engine. Such drainage holes are located upstream of the air filter since outside air can also enter through these drain holes. While this drainage arrangement has proven generally satisfactory, there are air cleaner casing designs where liquid can also collect on the downstream side of the air filter. Without drainage of this liquid, it could eventually be picked up by the filtered air and drawn into the engine which is, of course, undesirable.